Heretofore, there have been various proposals for electronically controlled fuel injection units that feedback control the air-fuel ratio of an engine intake mixture, using detection values from two oxygen sensors respectively disposed upstream and downstream of a three-way catalytic converter (exhaust gas purification catalytic converter) provided in an exhaust system for exhaust gas purification (Japanese Unexamined Patent Publication No. 4-72438).
Moreover, there has also been proposed diagnosis apparatus for determining deterioration of the catalytic converter (a drop in oxygen storage capacity of the catalytic converter) in the above mentioned air-fuel ratio feedback system using the two oxygen sensors, based on a comparison of the output frequencies of the upstream and downstream oxygen sensors.
However, when air-fuel ratio control or diagnosis control is carried out in this way, using the output of the oxygen sensor provided downstream of the catalytic converter, there is a proviso that the catalytic converter has attained an activation temperature.
More specifically, under conditions wherein the catalytic converter has not attained the activation temperature (non-active state), since the oxygen storage effect in the catalytic converter will not be sufficiently realized, there is the likelihood of an erroneous diagnosis of catalytic converter deterioration. Moreover, since the output of the downstream oxygen sensor is changed significantly by the influence from the catalytic converter immediately upstream, then if the catalytic converter is in the non-active state, there is the likelihood of air-fuel ratio control accuracy not being maintained.
Heretofore, conditions for estimating that the catalytic converter is sufficiently activated are first judged based on conditions such as, elapsed time from starting, and engine load, after which air-fuel ratio feedback control and catalytic converter deterioration diagnosis are initiated.
However, under actual vehicle travelling conditions, when the ambient temperature is low, and the vehicle is travelling at high speed, then it is possible for the catalytic converter, which has been at the activation temperature, to become cool and thus enter the non-active state. With the conventional method of judging the activated condition however, this condition, cannot be determined, so that air-fuel ratio control, and diagnosis, are carried out under conditions wherein the catalytic converter is not actually activated. As a result, there is the likelihood of deterioration in exhaust properties, and of erroneous diagnosis of catalytic converter deterioration.
As a technique for addressing the above problems, there is a method wherein the catalytic converter temperature is measured directly using a temperature sensor. However, since there are variations in temperature within the catalytic converter, it is difficult to determine the optimum location for the temperature sensor and to accurately detect the activated condition of the catalytic converter from the temperature sensor output. Moreover, the installation of a temperature sensor results in additional costs.